Anatomical Pulleys

Often, muscles are deflected by a bony prominence. When the direction of pull of a muscle is altered, the bony prominence causing the deflection forms an anatomical pulley. Pulleys change the direction without changing the magnitude of the force.¹

The change in direction of the action line of a muscle is away from the axis of the joint being crossed. The muscle action line will become more perpendicular to the contracting muscle fibers.¹

knee caps

While the magnitude of the force doesn’t change, by deflecting the action line away from the joint axis, the moment arm of the muscle force is increased. This change in direction of the muscle force results in improved ability of the muscle to generate torque, as torque is directly proportional to the magnitude of the force and the distance from the axis.¹²